Titanium and titanium alloy rod products are widely used for automobile parts such as engine valves and tools, because the specific strength and corrosion resistance of titanium and titanium alloy is excellent.
Because centerless grinding finish is required for such use of automobile parts materials, it is necessary to straighten the wire rod product to straightness less than 1 mm/m, i.e., the level of which centerless grinding can be conducted.
Since Young's modulus of titanium or titanium alloy is approximately a half of that of carbon steel or stainless steel, it is necessary to give a twice strain to straighten titanium and titanium alloys, comparing with carbon steel or stainless steel.
Therefore, when a wire rod made of titanium or titanium alloy is straightened at room temperature by using a conventional type of straightening apparatus used in the steel industry, the bend of the wire rod tends to remain. When a large amount of strain is given to the wire rod so as to remove the bend from it, cracks and flaws are likely to occur.
For example, a stainless steel wire rod is commonly straightened as follows. After the wire rod has been rolled, it is subjected to cold drawing and shaving in which the surface of the wire rod is shaved by an inverse die. After the surface of the wire rod has been finished, it is formed into a coil-shaped wire rod. Next, the wire rod is straightened by using a rotary housing type wire straightener and cut into a straight rod product.
When a wire rod made of titanium or titanium alloy is straightened by using the method described above, the following problems may be encountered. In order to straighten the wire rod of titanium or titanium alloy into a precise straight rod that can be subjected to centerless grinding, the straightening work of the wire rod of titanium or titanium alloy must be conducted at a higher accuracy than that of a stainless steel wire rod. Even if straightening work is conducted with high accuracy, problems of bend, cracks and flaws occur. As a result, the yield of the products becomes poor to cause the production cost higher.
Concerning a method of suppressing the occurrence of bends, cracks and flaws so as to enhance the yield of the products, the following methods have been disclosed. With respect to a deformed cross-section shape made of titanium alloy or super alloy, tensile straightening method in a movable radiation furnace is disclosed in Japanese Unexamined Patent Publication No. 2-187216. With respect to a sheet foil made of tungsten or molybdenum, tensile straightening method in a movable radiation furnace is disclosed in Japanese Unexamined Patent Publication No. 62-220220.
Most of straight rods of titanium or titanium alloy, which have been straightened, must be subjected to centerless grinding so that the surfaces of the rods can be finished. Therefore, the size, the amount of bend and scale thickness on the surfaces of the rods must satisfy small tolerances compared with that of shape steel and others.
Therefore, when the tensile straightening method mentioned above is applied to the straightening work in which a wire rod is straightened so as to make a straight rod, the wire rod is partially heated. As a result, it is difficult to provide a uniform contract of cross section in the longitudinal direction of the wire rod. Accordingly, the accuracy of the size in the longitudinal direction is lowered in one wire rod, and the ratio of defect rod increases in the straightening process.
Further, according to the above methods, only a predetermined tension is given to the wire rod, but an elongation caused by straightening is not stipulated. Accordingly, fluctuation of the target size increases according to the initial size of the rod, the distribution of temperature in the furnace and the fluctuation of the moving speed in the furnace. Therefore, it is difficult to conduct above method for a centerless grinding finished wire rod, or alternatively the number of centerless grinding operations increases. As a result, the productivity is deteriorated and the yield is lowered.
In addition to the above disadvantages, radiation heating causes surface oxidized scale which grows easily by oxidation. In order to take measures to solve the above problems, Japanese Unexamined Patent Publication No. 62-220220 discloses a method for coating to protect surface formed oxidation or a method for blowing inert gas into the furnace so as to lower the concentration of oxygen. However, because the affinity of titanium or titanium alloy with oxygen is strong, a surface coating to prevent oxidation shows little effective. Further a method for blowing inert gas is not economical, and the apparatus is complicated. As a result, the production cost is raised.
The object of the present invention is to produce straight rods of titanium or titanium alloy at a low cost by enhancing the yield of the product by means of hot-straightening methods under a specific straightening condition in order to reduce cracks and flaws and to stabilize the dimensional accuracy.
The other object of the present invention is to produce straightened titanium alloy rods suitably for automobile parts materials such as engine valves by applying hot-straightening method under a specific straightening condition.